Air pollution is a difficult problem to ignore, because those who live in cities — about two-thirds of the U.S. population — can see that ugly, ominous brown haze with their own eyes.

But the real danger lies in what you can’t see with the naked eye: the tiny compounds that are responsible for the worst health impacts from air pollution. A growing anthology of research is revealing just how dangerous these microscopic particles are.

Particulate matter (PM) involves a variety of substances and origins, from metals sent into the air by coal-fired power plants to plumes of pollen, smoke, and dust. On or near roadways, PM is everything from soot coming out of the tailpipe to bits of tire and brake pad that disintegrate in traffic.

Source: EPA

Particles are classified according to their size: coarse, fine and ultrafine. Coarse particles are between 2.5 and 10 micrometers (or microns) in diameter; Fine ones are 2.5 microns and smaller; ultrafine are sub-0.1 microns. For comparison, your basic human hair is between 50 and 70 microns thick, according to the illustration at right.

The larger particles “don’t get very far into our bodies, because they get trapped in the upper airways,” says Caleb Finch, who studies air pollution’s connections to aging as a professor at the University of Southern California’s Leonard Davis School of Gerontology.

Larger particles are trapped in the nasal passages, and slightly smaller ones make it as far as the lungs, where they can raise the risk of asthma and other respiratory disorders. But sub-2.0 particles pass through the lining of the lungs, into the bloodstream, and to the brain.

The damage done by these particles has been linked to heart disease, preterm births, autism, cancer and other illnesses. The work done at USC’s Davis School, as well as Keck School of Medicine and the USC Viterbi School of Engineering, has demonstrated how the smallest particles are linked to higher incidence of dementia, including Alzheimer’s, in older women.

Researchers analyzed the health records of 3,647 women age 65 to 79 in 48 states and found that those who lived in areas where PM2.5 levels exceeded federal safety standards had were 92 percent more likely to develop Alzheimer’s, compared with subjects who did not live in such polluted areas. The impact was even more pronounced in women who carried the APOE4 gene, a known risk-enhancer for Alzheimer’s.

USC scientists also exposed mice to fine air pollution particles captured on Los Angeles freeways for 15 weeks. Those mice had 60 percent more clusters of amyloid plaques, proteins that accelerate the cognitive decline associated with Alzheimer’s.

If you’re already running on academia overload, the Davis school made a video that sums up the findings:

Transportation is responsible for about half of air pollution, with much of the rest coming from stationary sources like coal-fired power plants, whose toxic compounds can travel for miles.

“Looking around the world, there are lots of places that have much worse pollution than L.A.,” Finch said. “We are much better than we were, but we have a long ways to go. … You can’t go to any city in the world without having some exposure.”

But long term, the best solution is to address our addiction to petroleum-based vehicle fuels. Using more alternatives to gasoline and diesel would have a tremendous benefit for air quality, and health.

We have met the enemy, and it is too small to see. But we have the tools we need to fight back.